net/ps3: Add support for gelic link negotiation

Add ethtool_ops.set_settings support to the PS3 gelic network driver.
Allows manual setting of ethernet link speed.

Signed-off-by: Hideyuki Sasaki <xhide@rd.scei.sony.co.jp>
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: David S. Miller <davem@davemloft.net>